The objective of the proposed research is to assess the contribution of ovarian hormones for the aggressive behavior that spontaneously emerges during pregnancy in the mouse. Pregnancy-induced aggression (PIA) is one aspect of maternal behavior that is thought to function for defense of the nest site and dispersal of conspecifics in growing populations. However, the physiological determinants of this behavior are unknown. One possibility is that estrogen (E) and progesterone (P) influence this aspect of maternal behavior. Therefore, in a series of 5 experiments we will examine the potential contribution of E and P (hormones known to maintain pregnancy in this species) for the display of aggressive behavior. Pregnant Rockland-Swiss Albino mice will be exogenously treated (via injection or constant-release silastic implants) with each of these steroids alone and with combined regimens designed to produce variable amounts of E and P in circulation. It is predicted that PIA will be facilitated when P is high relative to E, and inhibited when this ratio is reversed. In order to better evaluate the requirement of E and P for aggression during pregnancy, non-pregnant (i.e., pseudopregnant and ovariectomized virgin) mice will also be treated and examined for pregnancy-like aggressive behavior. Finally, other physiological and potential pharmacological effects of these regimens will be assessed by monitoring spontaneous homecage activity, temperature and body weight regulation in pregnant and non-pregnant animals. These proposed studies should provide new information on the contribution of ovarian hormones to intraspecific aggression in the female. They may also be relevant for our understanding of changes in mood and behavior that accompany alterations in the reproductive physiology of human females.